TIME: Almanac 1993/TIME Almanac 1993.iso/time/111191/1111320.000

Text File | 1992-08-28 | 3.5 KB | 83 lines

                                                                                SPACE, Page 78

The Big Blowup -- on Venus


New images from the Magellan probe suggest that the planet
orbiting closest to Earth is alive with volcanoes

By MICHAEL D. LEMONICK


    The surface of Venus has never seemed very hospitable.
Temperatures hover around 470 degrees C (900 degrees F), the
result of a runaway greenhouse effect, and the pressure of its
atmosphere, thick with carbon dioxide and sulfuric acid, is some
90 times that of Earth's. Lead would flow like water on Venus,
and water cannot have existed in liquid form for perhaps a
billion years.

    Now NASA's Magellan spacecraft seems to have found one
more horror in the nasty landscape: active volcanoes. Last week
the space agency released the first detailed map of Venus and
the most spectacular images ever made of its surface. The
pictures offer the best evidence to date that a planet once
presumed dead is actually a lively cauldron of geological
change.

    The most stunning image is of Venus' second tallest
mountain, Maat Mons, which rises 8 km (5 miles). Most of the
planet's many peaks, including 9.5-km- (6-mile-) high Maxwell
Montes, look bright in the radar pictures Magellan takes from
its orbit above the perpetual cloud cover. That means they are
strong reflectors of radar waves. But Maat Mons is dark; like
the Stealth bomber, it absorbs much of the radar falling on it.

    This intriguing fact, say project scientists, is a strong
hint that the mountain has recently been covered with lava.
Rock that sits on the surface of mountaintops appears to
weather quickly in the hot, chemically reactive atmosphere,
creating a soil that is rich in iron sulfide. It is this
mineral, the scientists believe, that shows up easily on radar.
If Maat Mons doesn't have any, it has presumably been
resurfaced, perhaps within the past few years.

    Such resurfacing has undoubtedly taken place in Venus'
lowlands: earlier images of the planet showed vast areas that
are remarkably free of craters. That would be easy to explain
on a planet like Earth, where cratering from meteor strikes is
erased by steady erosion. But while there is some evidence of
wind erosion on Venus, the best explanation for the lack of
cratering is periodic lava flows. Magellan has found direct
evidence of such flows, including domelike upwellings and
hardened streams of rock trailing down the sides of Venusian
peaks. There are also signs of other geologic activity,
including dramatic faulting and several distinct episodes of
mountain building. But until last week the evidence didn't
indicate whether the activity was still going on or had ceased
millions of years ago. The case for active Venusian volcanoes is
not yet proved, but Magellan, which is now well into its second
complete survey of the planet's surface, may eventually settle
the issue.

    While NASA studied Magellan's images, another space
explorer made history last week. Moving out beyond Mars, Galileo
became the first spacecraft to have a close encounter with an
asteroid. But pictures of the mysterious planetary fragment,
called Gaspra, are unavailable because Galileo's main antenna
for sending out images is frozen in the wrong position. Not
until 1992, when Galileo swings back by Earth, can smaller
antennas on the craft successfully transmit the missing
pictures. The frustrating delay makes scientists all the more
grateful for Magellan's reliable -- and revealing -- signals
from Venus.